1. Field of the Invention
This invention relates in general to digital communication signal processing techniques, and more specifically to modulation and demodulation techniques in multiple sub-channel communications systems.
2. Description of the Related Art
Multiple sub-channel modulation and demodulation engines, and corresponding transmitters and receivers, which are implemented with digital signaling techniques are known in the art. One application of such a device is to communicate several sub-channels of information within a single, broader bandwidth, channel. Such systems employ orthogonal frequency division multiplexed (“OFDM”) modulation, multi-carrier transmission (“MCT”) modulation, and others modulation techniques. The general modulation approach in such systems is to consolidate the multiple sub-channels as sub-carriers in a composite signal in the base-band, and then frequency-shift the consolidated signal to the allocated carrier signal band, usually centered about a carrier frequency. Although, it is also understood that such systems can operate in the base-band without frequency shifting, and that such systems can be utilized in other electromagnetic bands, such as infrared and visible light. Multiple sub-channel modulation and demodulation devices employ various topologies and architectures. Among these are a particular variety that employ a combination of inverse Fourier transforms (or Fourier transforms) with polyphase filters and a software commutator. This is typically accomplished within a digital signal processor, however software implementations in other kinds of computing devices are certainly available. The basic design parameters of such devices, that are used for the purpose of modulation, are characterized by a number of input sub-channel signals, or sub-carrier signals, that are sampled at an input, or base-band, sampling rate, and, that are spaced apart in frequency by an input frequency spacing that is ultimately translated through to a composite signal. In addition, such devices are characterized by the composite output signal that is sampled according to an output, or composite, sampling rate. Respecting demodulation, the same characteristics apply, but are related to the input sampling rate of a composite signal, and the output sampling rate and frequency spacing of plural output sub-channel signals. These characterizations of are of vital importance when considering an overall system design and architecture.
The design of a polyphase filter for band limiting each sub-channel according to its pass-band and frequency spacing is critical to the design of an efficient data communications system. So too is the communications baud rate, which is directly affected by the filter design and sub-channel spacing, and which is important to the spectral response of the composite signal transmitted in the allocated channel. Previously, those skilled in the art have understood that there were various constraints placed on the relationship between the input sampling rate, the sub-channel frequency spacing, and the output sampling rate. These constraints have been considered problematic respecting optimum system design, especially where maximum performance in marginal signal conditions were sought.
More particularly, polyphase filters operate by multiplying selected phases, or samples, of a filter impulse response with samples of one or more the aforementioned input signals. Prior-art multiple channel polyphase filters have synchronized the selected phases of the filter impulse response with the positions of a commutator of the filter. In prior-art multiple channel polyphase filters, a given position of the commutator has corresponded uniquely to a predetermined phase of the filter impulse response. Those of ordinary skill in the art of polyphase filter design have even accepted that in multiple sub-channel polyphase modulators, the sub-channel sampling rate must be an integral multiple of the input sample rate, which also implies that the channel bandwidth, or frequency spacing, must be an integer multiple of the input sample rate.
The tight limitations on the relationship between input sampling rate, frequency spacing, and output sampling rate were partially alleviated by the teachings of the present inventor, McCoy, in a prior U.S. Pat. No. 6,134,268 to McCoy for APPARATUS FOR PERFORMING A NON-INTEGER SAMPLING RATE CHANGE IN A MULTIPLE CHANNEL POLYPHASE FILTER (the '268 patent), the contents of which are hereby incorporated by reference thereto. The '268 patent teaches a multiple channel polyphase filter that includes a processing system for accepting and processing ‘M’ input channels of data, each sampled at an input sampling rate, wherein ‘M’ is a positive integer. The processing system is programmed to provide a commutator for the multiple channel polyphase filter, wherein the position of the commutator is decoupled from the phase of a filter impulse response selected for the position, thereby allowing the multiple channel polyphase filter to be operated at a sampling rate that is a non-integer multiple of the input sampling rate. The processing system is further programmed to operate the multiple channel polyphase filter at the non-integer multiple of the input sampling rate to obtain a non-integer sampling rate change. Other embodiments and applications taught by the '268 patent include a multiple channel polyphase filter, a multiple channel modulation engine, a corresponding multiple channel demodulation engine, a multiple channel transmitter, and a corresponding multiple channel receiver. While all of these embodiments teach the decoupling of the input sampling rate from the output sampling rate, each still suffers from a limitation defining the sub-channel frequency spacing by a fixed relationship between the input sampling rate and output sampling rate. Thus, even in view of the teachings of McCoy in the '268 patent, communication system designers are faced with a constraints on sub-channel spacing that is often times unable to deliver optimum performance in practical applications.
Thus, there is a need in the art for a modulation and demodulation engine applicable to multiple sub-channel systems that allows for the arbitrary specification of sub-channel frequency spacing with respect to input and output sample rates.